During the printing process, surface zones of the cylinders in a printing assembly move by rolling on one another. Since these surface zones are not themselves closed, but include channels in which the ends of a blanket or of a printing plate are securely clamped, contact pressure between the cylinders varies during the machine cycle. In particular, at high machine speeds, vibration is caused by the periodic appearance of imbalances and by the periodic variation in contact pressure. Such vibration can be seen in the printed image in the form of stripes, with the quality of printing being degraded because of variation in optical density.
Optimization, i.e. relatively high degrees of stabilization of contact pressure within one rotation of the machine is obtained by inserting "Schmitz rings", also known as "cords". These serve, advantageously, to stiffen the connections between cylinders in a printing assembly, without reaching permissible stress limits. The advantage of cords lies in increasing the frequency of the stripes and in reducing the amplitude of the stripes. However, at high speeds, stripes continue to appear, printing quality becomes unacceptable, and cords thus become inadequate.
Various devices are known in the state of the art for reducing twisting and bending vibration of cylinders in the print assemblies of a printing press. Document DE-C1-3 527 711 describes a print cylinder which includes a device for reducing twisting and bending vibration caused by channel overlaps by using at least one damping element disposed for this purpose in the cylinder of the print assembly. The damping element is effectuated by a transverse element fixed to the bottom portion of the envelope of said cylinder of the print assembly and by means of the shocks that occur in the gaps of the cylinder as it rolls over the channels. In addition, a point of contact is provided beneath the envelope of the cylinder on which the damping element can be effectuated in complementary manner while rolling on the channels.
Another structure for damping vibration in print cylinders is known from document DE-C1-4 119 825. A body that is symmetrical about the axis of rotation and that is positioned inside the cylinder forms a countermass to the envelope of the cylinder. As this internal body is symmetrical about the axis of rotation, it is surrounded by vibration-damping material. This structure thus provides a reduction in the amplitude of cylinder bending vibration which appears as a result of the shocks that take place in the gaps of the cylinder.
Document DE-C1-4 033 278 describes a bending vibration damper designed for a cylinder of a rotary printing press. A damper tuned over a broad frequency band is disposed in a special manner inside a cylinder of the print assembly, with the natural frequency of said damper corresponding to the frequency of oscillation of the cylinder of the print assembly. By having the damper deflect in phase opposition, the amplitude of bending vibration of the cylinder of the print assembly as induced by passing over the channels is reduced, as are higher harmonics thereof.